1. Field of the Invention
The present invention relates to a level transmitter for a liquid container, more specifically a fuel tank in a motor vehicle, having a plurality of sensors and a method for determining the level in the liquid container.
2. Description of the Related Art
Level transmitters for determining levels in liquid containers are much used in practice and are therefore known. Depending on the site of installation of the liquid container, the containers can take the most varied forms. For example, the liquid containers used in motor vehicles such as fuel tanks are adapted very strongly to prescribed spaces. Thus, many of the fuel tanks currently used have two or more chambers. A level transmitter is arranged in each of these chambers to determine a level in the liquid container having more than one chamber. In these cases, the level transmitters comprise float-actuated lever transmitters or transmitters comprising a plurality of sensors.
European Patent reference EP 0 690 293 A2 discloses level transmitters with magnetostrictive sensors for use in aircraft. In each case a level transmitter comprises a tube with a float arranged for vertical movement. A magnet is arranged in this float. Sensors made from magnetostrictive material are fastened in the tube at one end on a strip made from magnetic material. A wire-wound resistor is located at the opposite end of the sensors. If the float passes with the magnet into the region of the sensor, the sensor makes contact with the wire-wound resistor. A measuring unit arranged on the bottom of the fuel tank is used to determine the level in the fuel tank at the site of the respective level transmitter via the magnitude of the measured resistance. These transmitters are expensive because each level transmitter has a plurality of sensors, which are interconnected to form a sensitive measuring arrangement. Furthermore, each of these measuring arrangements must be encapsulated separately. Moreover, each level transmitter requires a float with a magnet, to transmit the liquid level to the sensors. Additional expenditure on assembly results from the fact that the tube must be fastened to the bottom of the fuel tank. Finally, each level transmitter is connected to its own dedicated measuring unit. In addition to the high initial outlay required for installing the multiplicity of level transmitters, there is a rise in the susceptibility to faults of such a system because of the number of components used. A further disadvantage in the use of plural level transmitters in a liquid container is that the useful container volume is reduced to an extent which cannot be neglected.
It is therefore an object of the present invention to provide a level transmitter and a method for determining the level which overcomes the problems of the prior art. The level transmitter according to the present invention is of a simple design and not susceptible to faults. Moreover, it is not to require any additional protective measures for electric lines or the evaluation unit. The level transmitter according to the present invention is to be capable of universal use, easy to assemble and cost-effective. The arrangement of the level transmitter is intended not to diminish the useful container volume substantially.
The object is achieved by a level transmitter for a liquid container, comprising, a plurality of sensors comprising a magnetostrictive material arranged within a liquid container, a transceiver unit operatively arranged for transmitting an alternating magnetic field and receiving mechanical oscillations produced by the plurality of sensors in response to the alternating magnetic field, said transceiver unit being arranged outside of the liquid container and, an evaluation unit operatively connected to said transceiver unit and arranged for receiving the measured oscillating values from the transceiver unit, wherein a dimension of each of said plural sensors differs from the dimensions of the others of said plural sensors such that each of the plural sensors has its own characteristic oscillatory behavior.
The level transmitter according to the present invention has a plurality of sensors made from a magnetostrictive material, a transmitter unit for transmitting an alternating magnetic field, a receiver unit for receiving mechanical, i.e., ultrasonic, oscillations, and an evaluation unit. The sensors are arranged in the liquid container. All other components of the level transmitter are located outside the liquid container. The transmitter applies the alternating magnetic field to the sensors to determine the level. Each sensor oscillates in response to the alternating magnetic field and the oscillations are detected in turn by the receiver unit. The measured oscillation values are fed thereafter to the evaluation unit. To determine whether the sensor is located in the liquid or in air, use is made of the circumstance that the oscillatory behavior of the sensor in the liquid differs from that in air.
If each of the sensors had the same physical dimensions, they would have the same oscillatory behavior and it would not be possible for the measured values to be assigned to the individual sensors by the evaluation unit. Accordingly, the dimensions of all the sensors differ in such a way that the oscillatory behavior of one sensor differs clearly from the others. By virtue of the fact that the frequency of the alternating magnetic field is adapted to the oscillatory behavior of each sensor, it is possible to measure the oscillatory behavior of each individual sensor. The measured oscillation values are fed to the evaluation unit, where each measured oscillation value is compared with a corresponding desired value, as a result of which the first signal for the level of the respective sensor is formed. After all the sensors have been interrogated, the evaluation unit likewise forms from the first signals a second signal which corresponds to the level in the liquid container.
An advantage of the level transmitter according to the present invention is that only the sensors are arranged in the liquid container. All the other components are located outside the liquid container. Since the sensors are a few square millimeters in size, and no electrical lines are required for making contact or for transmitting measured values to the sensors, the useful container volume is no longer reduced. Moreover, the level transmitter according to the invention comprises a very cost-effective design because one evaluation unit may be used to evaluate each of the sensors.
The fact that most of the components of the level transmitter are arranged outside the liquid container also keeps the cost of assembly particularly low. In some circumstances, the number of necessary separate openings in the liquid container may be economized.
The present invention also relates to a measuring method in which no forces arise, for example, from movements of floats or levers. Consequently, the fastening of the sensors in the liquid container is configured with particular ease. As stated above, the transceiver unit detects measured oscillation values without the use of cables. Accordingly, the site of installation of all the other parts of the level transmitter may be selected freely. This has the advantage, in turn, that the electrical components no longer have to be arranged in regions of danger to them, with the result that the susceptibility to faults is reduced, and the outlay for reliable operation is reduced.
The sensors may be mounted in a particularly simple way when they are fastened onto the container wall. The fastening, for example by bonding on or latching, may comprise any form if it is ensured that the sensors are not restricted in their oscillation. Owing to the arrangement at any desired points in the liquid container, the level may be determined exactly even in liquid containers of complicated shapes.
In a further embodiment, the sensors may be arranged on other built-in components in the liquid container. Thus, the outlay on assembly in the liquid container is eliminated when the sensors are mounted, for example, on the outside of a delivery unit or on a suction jet pump and are subsequently inserted with those components into the liquid container. For the purpose of an easier fastening of the sensors, the sensors may be arranged on a holder such that only the holder is to be fastened on the delivery unit.
The sensors are advantageously arranged in a fashion combined to form measuring points. Thus, the sensors arranged on the delivery unit, for example, form a first measuring point, while the sensors arranged on a suction jet pump form a second measuring point. In addition to these location-dependent measuring points, the sensors may be arranged at level-dependent measuring points. In this case, the sensors respectively form a measuring point at the highest and at the lowest level of the liquid container. Further measuring points may be arranged therebetween, depending on the resolution desired.
When the sensors are used in liquid containers with aggressive media, it is advantageous to cover the sensors with a protective coating, for example a film, it then being necessary to take account of the influence of the coating on the oscillatory behavior of the sensors.
Because the arrangement of the transceiver unit may be freely selected, its arrangement at a central point is particularly favorable for electrical measuring and control devices, since the electrical lines may be of particularly short design, and this leads to saving in weight and costs in addition to the better signal processing. However, it may also be advantageous in this case to arrange a transceiver unit separately in space.
The frequency of the alternating magnetic field is advantageously selected such that the sensors oscillate at resonant frequency. In another refinement of the method, a frequency range in which the resonant frequency of the sensor is situated, and which is traversed at intervals, is selected for each sensor. In this case, the resonant frequencies of the sensors in air or in the liquid may be selected as the resonant frequencies. The frequency, the amplitude and/or the decay time of the oscillation of the sensors are advantageously used in the evaluation unit to form the first signal which corresponds to the level at the sensor. Whether the sensor is situated inside or outside the liquid can be determined very easily by comparing the measured oscillation values with the prescribed values of the sensor stored in the evaluation unit.
The frequency of the alternating magnetic field is preferably selected between 20 and 100 kilohertz. Since levels in liquid containers frequently vary very slowly, in another embodiment of the method according to the present invention, the level is determined not continuously, but at intervals. The intervals may be a few milliseconds to a few minutes, such as, for example 1 millisecond to 10 minutes. Short measuring intervals are particularly advantageous whenever external influences, for example centrifugal forces, lead to falsified measurement results. For this purpose, the signals formed by the evaluation system are not relayed immediately as the level signal, but averaged over time. The signal for the level is subsequently formed from the mean value. Therefore, noise quantities may be effectively eliminated.
It must be ensured for the correct determination of level in the container that a measured oscillation value is also assigned to the correct sensor. In this case, it is of no significance whether the evaluation unit sends the transmitter a signal for the sensor to be interrogated, so that the transmitter thereupon emits the alternating field at the frequency characteristic of the sensor to be interrogated, or whether the sequence of the interrogation sensors in the transmitter and the evaluation unit is performed using a fixed program in a separately stored fashion.
The sequence of the interrogation of the individual sensors is arbitrary, and may be performed as dependent on location and as dependent on level. Particularly in the case of the level-dependent interrogation of measuring points, the outlay on measurement may be minimized by interrogating only the measuring point of interest as a function of the level determined. Thus, in the case of a relatively high level only the measuring points of the high level of the liquid container are interrogated. By analogy therewith, only the measuring points of the lowermost level are interrogated in the case of a low level.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.